Retinoids, metabolites of vitamin A, have been examined therapeutically against a variety of tumors, including gliomas. (Yung et al. (1996)) Nuclear receptor co-repressor (N-CoR) is closely associated with the retinoid receptor and is released upon ligand binding to the receptor. (Bastien et al. (2004)) By preventing the action of protein phosphatase-1 and protein phosphatase-2A, anti-phosphatases increase the phosphorylated form of N-CoR and promotes its subsequent cytoplasmic translocation. (Hermanson et al. (2002))
The phosphatase inhibitor, Cantharidin, has anti-tumor activity against human cancers of the liver (hepatomas) and of the upper gastrointestinal tract but is toxic to the urinary tract (Wang, 1989).
The publication of a report that cantharidin acts as a protein phosphatase inhibitor prompted a more general interest in compounds with this type of chemical structure (Li and Casida, 1992). Previously, it had been found that the simpler congener and its hydrolysis product (commercially available as the herbicide, Endothall) are hepatotoxic (Graziani and Casida, 1997). Binding studies have shown that the action of certain cantharidin homologs is direct on protein phosphatase-2A and indirect on protein phosphatase-1 (Honkanen et al., 1993; Li et al., 1993).
Of the known congeners of this type of compound, only the parent, cantharidin and its bis(normethyl)-derivative, norcantharidin, have seen any use as anti-cancer drug substances and only norcantharidin is used as an anti-neoplastic agent (Tsauer et al., 1997).
Despite these successes, few compounds of this type have been screened for anti-tumor or cytotoxic activity. Currently, there is a significant need to develop inhibitors of protein phosphatases that are more active, less toxic and more specific in action than the known substances mentioned above.